1. Field of the Invention
The present invention relates to a despreading circuit that receives a spread-spectrum signal and despreads the received signal at the receiving end, and an electronic apparatus incorporating the same.
2. Description of the Related Art
In recent years, communication services using High Speed Downlink Packet Access (HSDPA), a high-speed packet communication standard developed as an extension of W-CDMA (Wideband Code Division Multiple Access), have come to be provided as high-speed communication services for mobile telephones, personal computers, etc. In the services provided using HSDPA, different maximum communication speeds are defined for different service categories, and the communication speed changes according to the communication environment.
In HSDPA, each terminal receives packet data via a High Speed Physical Downlink Shared Channel (HS-PDSCH). Generally, in CDMA systems, one channel is coded by one spreading code, i.e., a channelization code, and is decoded by the same channelization code. In HS-PDSCH, however, data is split into a number of data. The split data are coded by a plurality of different channelization codes called multi-code. Since the data is split and coded for transmission, the number of data that can be simultaneously received at the terminal increases. This results in an improvement in communication speed.
For despreading of such multi-code spreading, use is often made of a Fast Hadamard Transform (FHT) circuit which performs computation between the received data and a Hadamard matrix. Despreading involves a matrix computation between a channelization code matrix and a received signal. The Hadamard matrix and the channelization code matrix coincide with each other if the rows of either one of the matrices are interchanged. This is the reason that the FHT circuit can be used for despreading.
The FHT circuit is efficient because a plurality of multi-code spread data can be decoded simultaneously. Accordingly, for HS-PDSCH, despreading is usually performed by using an FHT circuit.
On the other hand, as a pilot signal for receiving the HS-PDSCH, a primary common pilot channel (PCPICH) is also received at the same time. Since the PCPICH is not multi-coded, despreading is usually performed by using a despreader that applies the channelization code of the PCPICH, and that does not use an FHT circuit. Therefore, at the current state of the art, independent despreaders are used for HS-PDCH and for PCPICH, respectively. Using different despreaders in this way is not desirable because the amount of circuitry increases.
While a technique for reducing the amount of circuitry compared with the prior art, regardless of the number of code multiplexed signals, is proposed for use in a CDMA communication system (refer to Japanese Unexamined Patent Publication No. 2004-172650), the proposed technique is not one that solves the above problem.